How plastic makes corals sick

Microplastic spills the deep sea, plastic trash carpets swim in the ocean. And also the reefs are affected. Read more about the causes here.

Approximately 11.1 billion plastic parts are in the reefs of the Asian Pacific. This does not just look ugly – it’s a threat to the corals that are threatened anyway.

If all the plastic parts of the Pacific Ocean were combined, at least the area of ​​Western Europe would be the result. This is how the unimaginable quantities of plastic that meanwhile travel through our oceans are often described. In fact, oceanographers are not even able to say how much it is. Especially because plastic disintegrates in the sea over the years. Invisible – but still there – it drives through the ocean as a microplastic. But what effect does that have?

While scientists have yet to pinpoint exactly what effect microplastics have on the inhabitants of the oceans, there are numerous indications of how life-threatening larger pieces of plastic can be for many species. Sea turtles get caught in rubber rests or birds inadvertently eat plastic and starve to death with their stomach full of plastic.

And for corals too, the plastic is dangerous: for them, the risk of disease increases 20-fold when the reef is littered with plastic.

If the risk of disease under normal, plastic-free living conditions at about four percent, it rises by plastic parts that are in the reef, to 89 percent. This estimate comes from a study that biologists have now published in Science magazine (Lamb et al., 2018). For their study, the researchers studied 159 coral reefs in the Asian Pacific, on which live about 124,000 corals, and recorded how the plastic influences the living conditions of the coral.

They observed that the plastic, through natural movements and currents in the water, repeatedly comes into contact with the surface of corals. It bumps and rubs on the animals, injuring the top layer of tissue. Previous studies have shown that pathogens can more easily attack corals when the tissue is injured – such as the causative agent Halofolliculina corallasia, the causative agent of the skeletal eroding band disease (Coral Reefs, Page et al., 2007). An infection with H. corallasi is easily recognized by a thin, dark band that runs over the surface of a coral. Every day, it wanders a bit further, leaving behind a lifeless, white coral skeleton. This is how the scientists around biologist Joleah Lamb observed it. In particular, the disease skeletal eroding band occurred significantly more often when a coral reef was covered with many plastic parts.

Plastic donates dangerous shade

Injuries are not the only plastic problem. Tropical corals need a lot of light to get food through photosynthesis. A plastic bag, stuck between the corals, donates shade: The more bags, the darker it gets. Light and thus also food and oxygen become so scarce: All three are essential, however, that corals can fight against invading pathogens.

There are currently about 11.1 billion larger and smaller plastic parts on and in coral reefs in the Asian Pacific – in a region that hosts more than half of all coral reefs on Earth. The scientists estimate in their study that this amount will rise to nearly 16 million parts by the year 2025. The researchers call in their study to reduce the amount. With the many millions of people for whom coral reefs are an important livelihood through their diet or tourism, it is vital to “mitigate the outbreaks of disease to improve both human and ecosystem health,” they write their study.

There are currently about 11.1 billion larger and smaller plastic parts on and in coral reefs in the Asian Pacific – in a region that hosts more than half of all coral reefs on Earth. The scientists estimate in their study that this amount will rise to nearly 16 million parts by the year 2025. The researchers call in their study to reduce the amount. With the many millions of people for whom coral reefs are an important livelihood through their nutrition or tourism, it is vital to “mitigate the outbreaks of disease to improve both human and ecosystem health,” they write their study.